Q: How do I unlock the audio antitheft sytem on a 2005 Chevy Aveo? I disconnected the battery to clean the terminals. Do I need to take it to the dealer?

A: The code you need to enter should be in the glove box with the owner's manual.

Q: I have a convertible that I put away for 6 months a year. Will I have a problem with rust on my brakes in May when I bring the car out of the garage and put it on the road?

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A: When you start the car up, it'll make a grinding noise while decelerating as that light coat of rust is scrubbed off by the pad. Ignore this. Within a few blocks it will all be gone. If you start to feel a pulsating pedal, It's caused by a high spot where the discs were protected by the pad. If so, you'll need to get the brakes turned.

But unless you live right on the ocean where the salt flies in the air, it probably won't be a problem.

Next winter, every 6 weeks or so, pick a dry day and drive around for 30 minutes or so to prevent rust from accumulating, the fuel injection from gumming up and to keep the battery charged.

Q: I have a 1991 Mercedes Benz 300 D Turbo still going strong at 250,000 miles. I've had some engine work done, including valve replacement. The guy who did the work recommends that I add oil (just a few ounces) to my fuel mix when I fill up my tank to compensate for the new diesel fuel. The Mercedes dealer has never heard of this suggestion and does not agree. What's your take on this

A: I've never heard of this, so it's no wonder the M-B dealer hasn't, either. A quote from the Clean Diesel Fuel Alliance website:

"Like Low Sulfur Diesel fuel, ULSD fuel requires good lubricity and corrosion inhibitors to prevent unacceptable engine wear. As necessary, additives to increase lubricity and to inhibit corrosion are added to ULSD fuel prior to its retail sale. With these additives, ULSD fuel performs as well as Low Sulfur Diesel fuel."

Don't worry about it, in other words.

Q: My audio system started emitting a loud A/C like buzzing ever since I used a charger/booster to jump start the car after I installed a new battery in my 2000 Chrysler Concorde and left the positive terminal at the battery in a state of disconnection due to a bad terminal end. Do you have insight into what happened?

A: That AC-sounding buzz is just that. Alternators have diodes inside to convert the AC current from the stator into DC to operate the car and charge the battery. You've almost certainly blown a diode. This can easily happen if the battery is disconnected when the engine is running. Back EMF can spike the voltage in the stator windings to hundreds of volts, more than enough to fry one or more of the diodes. Back EMF? It's the same thing that lets an ignition coil generate the thousands of volts to rife the spark plugs. A coil of wire with current running through it has a magnetic field, and it's a pretty powerful one when the alternator is generating current. When the breaker points that feed 12 volts to the low-voltage windings in the ignition coil open, the magnetic field collapses, generating electric current as the lines of magnetic flux cross through the copper windings in the high-voltage windings. It has nowhere to go except to the high-voltage windings in the coil, which generate 20,000 or more volts to fire the plug.

Your alternator has no high-voltage windings. There is nowhere for the current being induced into the stator windings to go if the battery isn't in the circuit. So instead, the diode gets fried.

Your intermittent positive connection probably is what did it. A new battery should have had enough charge on it to start the car the car after one night idle. That says to me there was a bad connection at the positive terminal. Hooking up the battery charger and disturbing the clamp may have made enough connection to start the car, but wasn't good enough to keep from
blowing out the diode.

A decent technician, or even a decent tinkerer can test and replace the diode(s), but it might be cheaper to just replace the alternator with a rebuilt one.

Reader Tip: In your March 2008 issue, you had a reader with a problem of cold air blowing from the heater at idle. I just wanted to let you know that a co-worker of mine went through this recently, and it's a fairly common failure in German automobiles due to a shared electro-mechanical valve (AUDI is the German equivalent of General Motors) located on the firewall that regulates water into the heater core. Usually it's the solenoid that fails, and costs about $60 and 20 minutes to replace (basically, a more expensive, automotive version of the anti-siphon valves used in many automatic yard sprinkler systems). If it's the whole unit, it's slightly more involved, and $240 to replace. Either way, it's something the car owner can do pretty easily.